Medical imaging uses radiation to see inside the body without surgery, helping doctors diagnose broken bones, lung disease, kidney stones, and many other conditions. X-rays and CT scans use ionizing radiation, which can remove electrons from atoms and may slightly increase health risk at high doses. The goal of medical technology is to produce a useful image while keeping the radiation dose as low as reasonably achievable.
This balance is often summarized as image quality versus patient safety.
Radiation dose depends on the type of exam, exposure time, beam energy, patient size, and how many images are taken. Shielding, distance, and short exposure time reduce dose for patients and medical staff. Technologists often stand behind leaded glass or a protective barrier because radiation intensity decreases with distance and is blocked by dense materials.
The ALARA principle guides choices such as using the smallest useful beam area, avoiding repeat scans, and selecting ultrasound or MRI when they can answer the medical question without ionizing radiation.
Key Facts
- Radiation dose is the amount of ionizing radiation energy absorbed by tissue.
- Absorbed dose unit: 1 gray = 1 J/kg.
- Equivalent dose unit: sievert accounts for biological effect, H = D times wR.
- For X-rays, the radiation weighting factor is usually wR = 1, so 1 Gy corresponds to 1 Sv for equivalent dose.
- Inverse square law: intensity is proportional to 1/d^2, so doubling distance reduces intensity to one fourth.
- ALARA means keeping exposure as low as reasonably achievable while still getting the needed medical information.
Vocabulary
- Ionizing radiation
- Radiation with enough energy to remove electrons from atoms, which can affect molecules in living tissue.
- Radiation dose
- A measure of how much radiation energy is absorbed by the body or a specific tissue.
- ALARA
- A safety principle that means radiation exposure should be kept as low as reasonably achievable.
- Shielding
- Material such as lead, leaded glass, or concrete used to absorb or block radiation before it reaches people.
- CT scan
- A medical imaging method that uses many X-ray measurements from different angles to create cross-sectional images.
Common Mistakes to Avoid
- Thinking all medical scans use ionizing radiation is wrong because MRI and ultrasound do not use X-rays and are often chosen when they can provide the needed information.
- Ignoring distance from the source is wrong because radiation intensity follows the inverse square law, so even a small increase in distance can greatly reduce exposure.
- Assuming shielding makes radiation dose zero is wrong because shielding reduces exposure but may not block every photon, especially if it is thin or poorly positioned.
- Repeating an image without checking positioning and settings is wrong because unnecessary repeat exposures increase dose without improving patient care.
Practice Questions
- 1 A technologist moves from 1.0 m to 2.0 m away from an X-ray source. If the intensity at 1.0 m is 80 units, what is the intensity at 2.0 m?
- 2 A tissue sample absorbs 0.004 J of X-ray energy and has a mass of 0.20 kg. What is the absorbed dose in gray?
- 3 A doctor can use either a CT scan or an ultrasound to answer the same medical question. Explain which choice better follows ALARA and why.